Since moving into orbit about Mercury a little over one year ago, NASA's MESSENGER spacecraft has captured nearly 100,000 images and returned data that have revealed new information about the planet, including areas of permanent shadow at the poles that host the mysterious polar deposits.

One of the primary goals of MESSENGER's mission was to understand the nature of the radar-bright deposits at the poles of Mercury. The leading proposal since the deposits were discovered has been that radar-bright material consists dominantly of frozen water ice. Scientists have never had the imagery available before to see the surface where these radar-bright features are located. But images from MESSENGER's Mercury Dual Imaging System show that all the radar-bright features near Mercury's south pole are located in areas of permanent shadow, and near Mercury's north pole such deposits are also seen only in shadowed regions.

These results are consistent with the water-ice hypothesis but not definitive proof. But the MDIS image above, combined with ongoing analysis of data from MESSENGER's Neutron Spectrometer and the MLA, will provide a more complete picture of the nature of the deposits.